Coupling device for coupling a bone anchor to a rod and bone anchoring device with such a coupling device

ABSTRACT

A coupling device for coupling a bone anchor to a rod includes a receiving part having a longitudinal axis, a channel for receiving a rod, and an accommodation space for holding a head of a bone anchor, a pressure member having a first portion for holding and clamping the head and a deformable portion with a free end, and a locking member. The pressure member is expandable for inserting the head and adjustable to lock the head. When the deformable portion is at a first configuration, the free end of the deformable portion is supported by the receiving part, and at least part of the locking member is movable to exert a force on the deformable portion for adjusting the deformable portion to a second configuration where at least part of the deformable portion is farther away from the longitudinal axis compared to the first configuration.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.15/299,165, filed Oct. 20, 2016, which claims the benefit of U.S.Provisional Patent Application Ser. No. 62/244,498, filed Oct. 21, 2015,and claims priority from European Patent Application EP 15190858.9,filed on Oct. 21, 2015, the contents of which are hereby incorporated byreference in their entirety.

BACKGROUND Field

The invention relates to a coupling device for coupling a bone anchor toa rod, and further relates to a bone anchoring device including such acoupling device. The coupling device includes a receiving part with achannel for receiving a rod and an accommodation space for pivotablyholding a head of the bone anchor. The head can be locked at an angle byapplying pressure onto it via a pressure member. The pressure member hasa deformable portion. First, load is applied with a locking member tothe pressure member to deform the deformable portion to lock the head.Thereafter, the locking member comes into contact with the rod and locksthe rod. The coupling device permits insertion of the bone anchor from abottom end of the receiving part, thus forming a modular polyaxial boneanchoring device that allows locking of the head and the rod in asequential manner using a tool with a single drive portion that engagesthe locking member.

Description of Related Art

US 2013/0345761 A1 describes a polyaxial bone anchoring device includinga bone anchoring element with a shank and a head in a receiving partpivotably coupled to the shank and having a channel for receiving a rod.A pressure member is arranged in the receiving part and is configured toexert pressure onto the head to lock the head in the receiving part. Thepressure member has a deformable portion. Load applied to the pressuremember by a locking member clamps the head, and thereafter the lockingmember contacts the pressure member such that the deformable portion isdeformed and the locking member comes into contact with the rod andclamps the rod.

SUMMARY

While the above device allows the locking of the head in the receivingpart first and thereafter the locking of the rod with a singleinstrument in a sequential manner, the necessary locking force to beapplied is high. This results from the fact that the final step ofclamping and locking the rod involves the deformation of a deformableportion of the pressure member. The force necessary to effect thedeformation may not contribute to and can be lost for the clamping andlocking of the rod. To compensate for this, a relatively high force hasto be applied for locking the rod.

Embodiments of the invention provide a coupling device for coupling arod to a bone anchor, and a polyaxial bone anchoring device with such acoupling device that allows for sequential locking of the head and therod, for example, with an instrument having a single drive portion,where the device has increased versatility with respect to clinicalapplications.

The coupling device includes a pressure member that exerts pressure ontothe head and a deformable portion with a free end that is supported inthe receiving part. With a locking member, a force is exerted onto thedeformable portion and deforms the deformable portion from a firstconfiguration to a second configuration, wherein the pressure exerted bythe pressure member onto the head increases. By means of this, thedeformable portion functions similar to a toggle lever that transformsfrom an angled configuration into a less angled or substantiallystraight configuration. For locking the rod, the locking member isfurther advanced and tightened, without further deformation of thedeformable portion of the pressure member. Therefore, loss of force dueto deformation of the pressure member during locking of the rod isreduced. This permits higher clamping forces acting onto the rod, and inturn increases the safety of the locking.

The coupling device together with a bone anchor forms a polyaxial boneanchoring device of the bottom loading type, wherein the head of thebone anchoring element can be inserted into the receiving part from thebottom side, or from the side opposite to the locking member. Therefore,a modular system can be provided that allows combinations of a varietyof bone anchors with the coupling device to form a polyaxial boneanchoring device.

With a polyaxial bone anchoring device according to embodiments of theinvention, a sequential locking where first the head is locked andthereafter the rod is fixed by the application of a single instrumentwith a single drive portion is possible. This facilitates easierhandling of the polyaxial bone anchoring device during surgery.

In some embodiments, an optional rod retention member permits holding ofan inserted rod, such that the rod cannot move out of the channel evenwhen the rod is not yet locked by the locking member. This can alsofacilitate easier handling of the polyaxial bone anchoring device duringsurgery. The polyaxial bone anchoring device including the rod retentionmember may be of a bottom loading type, or may be of a top loading type,wherein the bone anchoring element is inserted into the receiving partfrom the top side, or the same side into which the locking member isinserted.

In some embodiments, a shape of the pressure member and optionally ofthe rod retention member is such that the polyaxial bone anchoringdevice can be used with rods having different diameters. Thus, theversatility of the polyaxial bone anchoring device can be furtherincreased.

In some embodiments, the deformable portion of the pressure member iselastically deformable and includes a highly elastic material, forexample, a material with superelastic properties, such as anickel-titanium (NiTi)-alloy, in particular Nitinol, or, for example,Beta-titanium. In these embodiments, due to the high elasticity of thedeformable portion, a loss of force during deformation can be furtherreduced. In addition, due to the elasticity of the deformable portion,size tolerances of the pressure member and of the parts interactingtherewith can be more balanced.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom the description of embodiments by means of the accompanyingdrawings. In the drawings:

FIG. 1 shows a perspective exploded view of a polyaxial bone anchoringdevice according to an embodiment of the invention, together with rodshaving different diameters.

FIG. 2 shows a perspective view of the polyaxial bone anchoring deviceaccording to FIG. 1 in an assembled state with an inserted rod.

FIG. 3 shows a perspective view from a top of a receiving part of thepolyaxial bone anchoring device according to FIGS. 1 and 2.

FIG. 4 shows a perspective view from a bottom of the receiving part ofFIG. 3.

FIG. 5 shows a top view of the receiving part of FIGS. 3 and 4.

FIG. 6 shows a cross-sectional view of the receiving part of FIGS. 3 to5, the cross-section taken along line A-A in FIG. 5.

FIG. 7 shows a perspective view from a top of a pressure member of thepolyaxial bone anchoring device of FIGS. 1 and 2.

FIG. 8 shows a perspective view from a bottom of the pressure member ofFIG. 7.

FIG. 9 shows a top view of the pressure member of FIGS. 7 and 8.

FIG. 10 shows a cross-sectional view of the pressure member of FIGS. 7to 9, the cross-section taken along line B-B in FIG. 9.

FIG. 11 shows a perspective view from a top of a rod retention member ofthe polyaxial bone anchoring device according to FIGS. 1 and 2.

FIG. 12 shows a perspective view from a bottom of the rod retentionmember of FIG. 11.

FIG. 13 shows a top view of the rod retention member of FIGS. 11 and 12.

FIG. 14 shows a cross-sectional view of the rod retention member ofFIGS. 11 to 13, the cross-section taken along line D-D in FIG. 13.

FIG. 15 shows a perspective exploded view of a locking member of thepolyaxial bone anchoring device of FIGS. 1 and 2.

FIG. 16 shows a cross-sectional view of the locking member of FIG. 15 inan assembled state, the cross-section taken in a plane including acentral axis C of the locking member.

FIG. 17a shows a cross-sectional view of the polyaxial bone anchoringdevice of FIG. 2 with an inserted rod having a first diameter in anassembled and locked condition, the cross-section taken in a planeincluding a central axis of the device and extending perpendicular to alongitudinal direction of the inserted rod.

FIG. 17b shows a cross-sectional view of the polyaxial bone anchoringdevice of FIG. 2 with an inserted rod having a second diameter, smallerthan the first diameter, in an assembled and locked condition, thecross-section taken in a plane including a central axis of the deviceand extending perpendicular to a longitudinal direction of the insertedrod.

FIGS. 18a to 18d show perspective views of steps of assembling acoupling device of the polyaxial bone anchoring device of FIGS. 1 and 2.

FIGS. 19a to 19d show cross-sectional views of steps of inserting a headof the bone anchoring element into the receiving part of the couplingdevice when assembling the polyaxial bone anchoring device.

FIGS. 20a to 20d show cross-sectional views of inserting a rod into thepolyaxial bone anchoring device and locking the rod and the headrelative to the coupling device.

DETAILED DESCRIPTION

A polyaxial bone anchoring device according to a first embodiment isshown in FIGS. 1 and 2. The polyaxial bone anchoring device includes ananchoring element 1 having a shank 2 with a threaded portion and a head3. The head 3 has a spherically-shaped outer surface portion and, on itsside opposite to the shank 2, a recess 4 for engagement with a tool. Acoupling device including a receiving part 5 is provided for couplingthe bone anchoring element 1 to a rod 100 or 101. The polyaxial boneanchoring device in FIGS. 1 and 2 is of the bottom-loading type, or inother words, the bone anchoring element 1 can be inserted into thereceiving part 5 from a bottom end of the receiving part 5. In addition,the coupling device includes a pressure member 6 configured to bearranged in the receiving part 5 to exert pressure onto the head 3 ofthe bone anchoring element 1. To hold an inserted rod temporarily inplace, an optional rod retention member 7 can be provided. For lockingthe head 3 and the rod 100 or the rod 101 relative to the receiving part5, a locking member 8 is provided that is insertable into the receivingpart 5. The coupling device is configured to be used selectively withthe rod 100 having a first diameter, or with another rod 101 having asecond diameter different from the first diameter, for example a smallerdiameter.

Referring further to FIGS. 1 to 6, the receiving part 5 has a first endor top end 5 a and a second end or bottom end 5 b, and has asubstantially cylindrical construction, with a longitudinal axis C thatgoes through the top end 5 a and the bottom end 5 b. Coaxial with thelongitudinal axis C, a coaxial first bore 51 is provided that extendsfrom a distance from the top end 5 a to a distance from the bottom end 5b. A hollow space that is substantially rotationally symmetrical isformed between the bottom end 5 b and the first bore 51, which serves asan accommodation space 52 for the head 3. The accommodation space 52 hasa bottom opening 52 a at the bottom end 5 b with an inner diameter thatis greater than an outer diameter of the head 3. This permits insertionof the head 3 through the opening 52 a from the bottom end 5 b. An innerdiameter of the accommodation space 52 at the junction to the first bore51 is smaller than a diameter of the first bore 51, such that a shoulder52 b is formed at the junction. The accommodation space 52 further has agreater diameter approximately midway between the junction to thecoaxial bore 51 and the opening 52 a. A size of the accommodation space52 is such that the pressure member 6 can be expanded therein when thehead 3 is inserted. Adjacent to the opening 52 a, there is a narrowingportion 52 c that is configured to engage a narrowing portion of thepressure member 6. The narrowing portion 52 c may have a tapered shapeor a spherical segment shape or any other shape that permits compressionof a lower end of the pressure member 6. An edge 52 d may be formed at adistance from the bottom end 5 b that protrudes into the accommodationspace 52 and contributes to the compression of the pressure member 6.

Between the top end 5 a and the first bore 51, a coaxial second bore 53is provided that has a smaller inner diameter than the first bore 51.The receiving part 5 further comprises a U-shaped recess 54 starting atthe top end 5 a and extending in the direction of the bottom end 5 b. Bymeans of the U-shaped recess 54, two free legs 55, 56 are formed thatdefine a channel for receiving the rod 100 or the rod 101. Adjacent tothe top end 5 a an internal thread 57 is provided at the inner surfaceof the legs 55, 56 in the region of the second bore 53. In theembodiment shown, the internal thread 57 is a flat thread havingsubstantially horizontal upper and lower thread flanks. However, anyother thread form can be used for the internal thread 57. As depictedmore in detail in FIGS. 3, 4 and 6, recesses 53 a, 53 b are respectivelyprovided at the legs 55, 56 below a lower side of an uppermost threadturn of the internal thread 57 to accommodate a portion of the pressuremember 6 therein, so that a first abutment or stop 57 a for a portion ofthe pressure member 6 is provided by the lower side of the uppermostthread turn of the thread 57.

As depicted in particular in FIGS. 3 and 6, two recesses 58, 59 arerespectively formed at an inner wall of the legs 55, 56 in the region ofthe coaxial second bore 53, and are configured to accommodate a portionof the pressure member 6 therein. The recesses 58, 59 are respectivelylocated at substantially the center of each leg 55, 56 in acircumferential direction, at a lower portion of the thread 57, and openinto the first bore 51 as well as into the recesses 53 a, 53 b describedabove. An upper wall or a corner of each of the recesses 58, 59 forms asupport 58 a, 59 a for the pressure member 6. Simultaneously, thissupport 58 a, 59 a forms a second stop for preventing the pressuremember 6 from moving towards the top end 5 a when the pressure member 6is in a pre-locking position. Moreover, in order to facilitate theplacement of the pressure member 6, a shallow groove 58 b, 59 b isformed on the inner wall of each leg 55, 56, respectively, at the regionof the U-shaped recess 54 at either side of the channel and atapproximately the height of the supports 58 a, 59 a.

Moreover, through-holes 58 c, 59 c may be provided in the receiving part5 that extend in a transverse direction through walls of the receivingpart 5 and into the recesses 58, 59, respectively. The through-holes 58c, 59 c may facilitate application of an instrument for acting onto aportion of the pressure member 6.

Referring now to FIGS. 1 and 7 to 10, the pressure member 6 has asubstantially cylinder segment-shaped first portion 61 with two sideflanges 62, 63 provided on opposite sides of the first portion 61 andalso having cylindrical outer surface portions. At a top surface 6 a ofthe pressure member 6 between the flanges 62, 63, a rod supportingsurface 64 is provided in the form of a substantially V-shaped recesswith a longitudinal axis perpendicular to a central axis C that alignswith the central axis C of the receiving part 5 when a pressure member 6is mounted to the receiving part 5. The rod supporting surface 64 isconfigured to support rods 100, 101 of different diameters. In addition,a diameter of an inserted rod is between a maximum and a minimumdiameter, the rod can always be safely supported by the rod supportingsurface, for example, along two longitudinal contact lines. The pressuremember 6 further includes on a side opposite to the top surface 6 a asecond portion 65 that is recessed with respect to the first portion 61.The second portion 65 has a hollow interior 66, which is substantiallyspherically shaped to clamp the spherical head 3 therein and is open ata second end 6 b of the pressure member 6. An outer wall of the secondportion 65 includes a first portion 65 a that is substantially sphericaland a narrowing portion 65 b that narrows towards the second end 6 b.The narrowing portion 65 b is configured to cooperate with the narrowingportion 52 c of the receiving part 5. The narrowing portion 65 b may betapered or spherical or shaped in another manner, and may notnecessarily correspond to the shape of the narrowing portion 52 c of thereceiving part 5, as long as the narrowing portion 65 b can becompressed by the narrowing portion 52 c of the receiving part 5. Thesecond portion 65 of the pressure member 6 further has a plurality ofslits 67 extending from the free end 6 b through at least part of thesecond portion 65. A number and dimensions of the slits 67 are such thata wall of the second portion 65 is flexible enough to snap onto the head3 when the head 3 is being inserted. The slits 67 may have enlargedportions 67 a at their closed ends. A coaxial bore 61 a extends throughthe first portion 61 into the hollow interior 66 for providing access tothe screw head 3 by an instrument.

As shown in the figures, two upstanding arms 68, 69 protrude from theupper surface 6 a of the first portion 61 of the pressure member 6. Thearms 68, 69 are respectively positioned substantially at the middle ofthe upper surface 6 a of the flanges 62, 63, and each arm 68, 69 has asubstantially rectangular cross-section. In the embodiment shown, a longside of the rectangle is substantially parallel to a longitudinal axisof the rod supporting surface 64 and is smaller than a width of theflanges 68, 69 in this same direction. A width of each of the arms 68,69 in a direction perpendicular to the longitudinal axis of the rodsupporting surface 64 is also smaller than the width of the flanges 68,69 in the same direction, so that the arms 68, 69 are spaced apart fromthe edges of the flanges 62, 63. In addition, a thickness of the arms68, 69 may decrease in an upward direction, away from the first portion61, to facilitate flexion.

As further shown in particular in FIGS. 7 to 10, each arm 68, 69 has afree end 68 a, 69 a. Adjacent to the upper surface 6 a, each of the arms68, 69 has a substantially vertically extending long portion 68 b, 69 b.From the end of the long portions 68 b, 69 b, short portions 68 c, 69 cextend radially outward and away from the long portions 68 b, 69 b at anangle of substantially 90° or greater. The long portions 68 b, 69 b maybe twice as long as the short portions 68 c, 69 c, or longer. A lengthof the arms 68, 69 is such that the long portions 68 b, 69 b extendabove an upper surface of a rod that is placed onto the rod supportsurface 64. The long portions 68 b, 69 b may be slightly curved in anoutward direction, i.e. away from the central axis C. As can be seen inparticular in FIG. 10, the arms 68, 69 may be separate parts that may bemounted to the first portion 61 of the pressure member 6. Morespecifically, the arms 68, 69 may be inserted in corresponding recesses6 c in the upper surface 6 a of the first portion 61 of the pressuremember 6.

The arms 68, 69 may be elastically deformable in such a manner that theycan be deflected outward by an applied load, and can return to theiroriginal shape and position once the load is relieved or removed. Inparticular, in a first configuration, the arms 68, 69 are in theirresting position that is defined by the bent shape wherein the shortportions 68 c, 69 c form an obtuse angle with the long portions 68 b, 69b. In a second configuration, the arms 68, 69 are deformed such that theangle between the short portions 68 c, 69 c and the long portions 68 b,69 b is increased, and the long portions 68 b, 69 b may be deflectedoutward. In the second configuration, a load can be transferred throughthe arms 68, 69 onto the head 3. In other words, the pressure member 6can behave similar to a toggle lever formed by the arms 68, 69 that actbetween the receiving part 5 and the head 3.

Referring to FIGS. 1 and 11 to 14, in this embodiment, the rod retentionmember 7 is a separate part that is configured to be connected to thepressure member 6. The rod retention member 7 has a generallycylindrical base portion 71 with a top surface 7 a and an oppositebottom surface 7 b. At the top surface 7 a, a recess configured tosupport the rod is formed with a substantially cylindrical first section72 a having a first radius and a substantially cylindrical secondsection 72 b having a second radius smaller than the first radius.Further, a cut-out 73 is provided that matches the shape of the rodsupporting surface 64 of the pressure member 6 such that, as depicted inFIG. 18a , when the rod retention member 7 is mounted to the pressuremember 6, the rod supporting surface 64 of the pressure member canextend through the cut-out 73. From the top surface 7 a, a pair of arms74 a, 74 b, 75 a, 75 b extend upward on either side of the rodsupporting surface 72 a, 72 b when seen along a rod axis. An innersurface of the arms 74 a, 74 b, 75 a, 75 b is substantially cylindrical,with a radius that may be adapted to a radius of a particular rod. Adistance between the inner surface of opposite arms 74 a, 75 a, or 74 b,75 b is configured such that the inserted rod can be held tightlytherebetween. A height of the arms 74 a, 74 b, 75 a, 75 b in the axialdirection is such that when one of the insertable rods is insertedbetween the arms 74 a, 74 b, 75 a, 75 b and rests on the rod supportingsurface 64, the arms 74 a, 74 b, 75 a, 75 b are configured to clamp theinserted rod between them.

The arms 74 a, 74 b, 75 a, 75 b may each also be curved in an outwarddirection and may have substantially a square-shaped cross-section. Asshown in the embodiment, the arms are shorter in length compared to thearms 68, 69 of the pressure member 6. A distance between each pair ofarms 74 a, 74 b or 75 a, 75 b, respectively, is such that the arms 68,69 of the pressure member 6 can be placed or positioned at leastpartially between the corresponding arms 74 a, 74 b, 75 a, 75 b of therod retention member 7. At the bottom surface 7 b, a substantiallycylindrical recess 76 is provided, the depth of which is such that therod retention member 7 can be placed onto the first portion 61 of thepressure member 6 in such a manner that the bottom surface 7 b of therod retention member is substantially flush with a bottom side of thefirst portion 61 of the pressure member 6, as can be seen, for example,in FIG. 18b . On an outside of the arms 74 a, 74 b, 75 a, 75 b, a firstrecess 77 a, 78 a is respectively formed in the top surface 7 a forpermitting the arms 68, 69 of the pressure member 6 to extendtherethrough. A further recess 77 b, 78 b is provided in the bottomsurface 7 b that is in communication with the first recess 77 a, 78 aand the recess 76 and permits the flanges 62, 63 of the pressure member6 to extend therethrough, as shown in FIGS. 18a and 18 b.

The arms 74 a, 74 b, 75 a, 75 b are resilient in a manner such that theycan be deflected outward and snap back to their resting position, forexample, as illustrated by the full lines and the dashed lines in FIG.14.

Referring now in particular to FIGS. 1 and 2 as well as FIGS. 15 and 16,the locking member 8 can be a two-part locking member that includes aset screw 81 and a washer 84 rotatably connected to the set screw 81.The set screw 81 is configured to engage the internal thread 57 of thelegs 55, 56 and has an engagement portion 82 for a driver on its upperside. A through-hole 83 may be provided on the lower side of the setscrew 81. The washer 84 may be connected to the set screw 81 with acentral coaxial pin 85 that extends through the through-hole 83, and forexample, that can be deformed on the other side of the through-hole 83to provide a rivet-like connection. A lower edge 84 a of the washer 84may be rounded. This ensures a smooth sliding contact between thelocking member 8 and the pressure member 6. The washer 84 has asubstantially rectangular main portion 86 and two oppositecylinder-segment shaped flanges 87, 88 that extend from the main portion86 on opposite sides. The flanges 87, 88 each has a substantiallyvertically extending substantially flat outermost surface portion 87 a,88 a with a width in a direction perpendicular to the central axis thatis substantially the same or greater than the width of the arms 68, 69of the pressure member 6. By means of this, the washer 84 can smoothlyslide along the inner surface of the arms 68, 69 of the pressure member6 when the locking member 8 is advanced in the receiving part 5. Thesubstantially flat vertical surface portions 87 a, 88 a allow for abetter transfer of the pressure force applied by the washer 84 onto thearms 68, 69. The rectangular main portion 86 prevents the washer 84 fromrotating when the set screw 81 is screwed into the receiving part 5.Therefore, occurrences of transverse forces acting onto the arms 68, 69is avoided.

The receiving part 5, the pressure member 6, the rod retention member 7,the bone anchoring element 1, and the locking member 8 can each be madeof any body-compatible material, such as a body-compatible metal ormetal alloy, for example, stainless steel, titanium, or magnesium, or ofa body-compatible plastic, such as polyether ether ketone (PEEK) orpoly-L-lactide acid (PLLA). The respective parts can all be made of thesame or of different materials. In particular, the entire pressuremember 6 may be made of a material exhibiting high elasticity, such as anickel-titanium alloy exhibiting super-elastic properties, for exampleNitinol, or Beta-titanium. Alternatively, only the arms 68, 69 of thepressure member 6 can be made of such a highly elastic material. The rodretention member 7 can be made of a body-compatible plastic material asmentioned above, for example, PEEK, while the remaining parts can bemade of a metal or metal alloy as mentioned above.

Referring to FIGS. 18a to 18d , the coupling device may be pre-assembledas follows. In a first step, as depicted in FIG. 18a , the rod retentionmember 7 is mounted onto the pressure member 6 such that the rodsupporting surface 64 extends through the cut-out 73 in a manner suchthat an inserted rod rests only on the rod supporting surface 64 of thepressure member 6 and not on the surfaces 72 a, 72 b of the rodretention member 7. The arms 74 a, 74 b extend to the left and to theright of the arm 68 of the pressure member 6. The arms 75 a, 75 b extendto the left and to the right of the arm 69 of the pressure member 6. Theassembly of the pressure member 6 and the rod retention member 7 is theninserted into the receiving part 5, as illustrated in FIGS. 18b and 18c. To accomplish this, the assembly is orientated relative to thereceiving part 5 in a manner such that the arms 68, 69 of the pressuremember 6 are aligned with the U-shaped recess 54 of the receiving part5. The assembly is moved downward until the free ends 68 a, 69 a of thearms 68, 69 of the pressure member 6 are beneath the first stop 53 a onthe upper portion of the legs 55, 56. Thereafter, the assembly isrotated, as depicted in FIG. 18d , until the arms 68, 69 of the pressuremember can extend into the second bore 53 of the receiving part 5.

Steps of assembling the bone anchoring device are now explained withreference to FIGS. 19a to 19d . In FIG. 19a , the pressure member 6 isat an insertion position in which the head 3 of the bone anchoringelement can be inserted into the hollow interior 66 of the cap-likesecond portion 65. The free ends 68 a, 69 a of the arms 68, 69 abutagainst the first stop 53 a provided in the receiving part 5. In thisposition, the arms 68, 69 are slightly compressed inwards at theirjunction between the long portions 68 b, 69 b and the short portions 68c, 69 c. The pressure member 6 can not be pushed further upward duringinsertion of the head 3. The arms 74 a, 74 b, 75 a, 75 b of the rodretention member 7 extend slightly into the rod receiving channel of thereceiving part 5 due to their curved shape. The cap-like second portion65 of the pressure member 6 is arranged in the accommodation space 52.

As shown in FIG. 19b , the head 3 of the bone anchoring element 1 isthen inserted through the open end 6 b into the hollow interior 66 ofthe pressure member 6. Due to the flexibility of the wall of thecap-like second portion 65 provided by the slits 67, the cap-like secondportion 65 expands in the accommodation space 52 and snaps onto the head3, as shown in FIG. 19c . In the insertion position, the head 3 ispivotable in the receiving part 5, and is also removable if the pressuremember 6 remains in this position.

As illustrated in FIG. 19d , a downward movement of the pressure member6 with the rod retention member 7 allows the arms 68, 69 to spread oncethey engage the recesses 58, 59. The free ends 68 a, 69 a of the arms68, 69 abut against the second stop 58 a, 59 a. By the downwardmovement, the pressure member 6 with the inserted head 3 passes the edge52 d, and the narrowing portion 52 c of the receiving part 5 and thenarrowing portion 65 b of the pressure member 6 engage with one another.By means of this, head 3 is held in the receiving part 5. This positionof the pressure member 6 as depicted in FIG. 19d is a pre-lockingposition, where the head 3 is still pivotable but cannot be removedthrough the lower opening 52 a of the receiving part 5. The pre-lockingposition is secured by the second stop 58 a, 59 a.

It shall be understood that, in some embodiments, the engagement of thehead 3 of the anchoring element 1 with the inner wall of the cap-likesecond portion 65 of the pressure member 6 can be a frictionalengagement, so that in the insertion position or even more in thepre-locking position, the head 3 is pivotable when the friction force isovercome. This allows maintaining of the receiving part 5 relative tothe bone anchoring element 1 in an adjustable angular position.

In use, the bone anchoring device can be, for example, pre-assembled insuch a way that a suitable bone anchoring element 1 is selected andinserted into the receiving part 5 with inserted pressure member 6 andoptionally inserted rod retention member 7. The bone anchoring devicecan be brought into the pre-locking position by slightly pushing downthe pressure member 6 relative to the receiving part 5. Then, the boneanchoring element 1 can be implanted into a bone. In an alternativemanner, the bone anchoring element 1 is first implanted into the bone,and thereafter the coupling device is mounted onto the head 3 of thebone anchoring element 1.

Next, the connection of the polyaxial bone anchoring device with a rodwill be described with reference to FIGS. 20a to 20d . Usually, at leasttwo bone anchoring elements 1 are inserted into bone parts or adjacentvertebrae, and the corresponding receiving parts 5 are aligned toreceive the rod 100. Then, the rod 100 is inserted from the top end 5 aof the receiving part 5 into the channel provided by the U-shaped recess54 of the receiving part 5. Once the rod 100 passes the arms 74 a, 74 b,75 a, 75 b of the rod retention member 7, the arms 74 a, 74 b, 75 a, 75b are deflected outward and, due to their resiliency, snap back or justexert pressure to clamp the rod 100 by friction, as depicted in FIG. 20b. This allows an easier handling of the locking member 8 later, becausewobbling of the rod 100 in the receiving part is prevented.

Next, as depicted in FIG. 20c , the locking member 8 is screwed inbetween the legs 55, 56 of the receiving part 5. When the locking member8 is further screwed in, the lower rounded edge 84 a of the washer 84contacts the short portions 68 c, 69 c of the arms 68, 69 near thetransition to the long portions 68 b, 69 b. Because the washer 84 isrotatably connected to the set screw 81, the washer 84 basically doesnot rotate when the set screw 81 is further advanced into the receivingpart 5. This results in the forces exerted by the locking member 8 ontothe arms 68, 69 being substantially axial, where radial forces andtransverse forces are reduced. In the configuration shown in FIG. 20c ,the rod 100 is still movable in a longitudinal direction along the rodaxis because the locking member 8 is not yet in contact with the rod100.

The locking member 8 exerts a force onto the arms 68, 69 that deformsthe arms 68, 69, as can be seen in FIG. 20d . Because the free ends 68a, 69 a of the arms 68, 69 are supported in the receiving part 5 and thepressure member 6 with the inserted head 3 is supported in the narrowingportion 52 c of the receiving part 5, there is limited or no space forthe arms 68, 69 to extend in the axial direction, so the long portions68 b, 69 b of the arms 68, 69 are deflected radially outward, asdepicted in FIG. 20d . The deformation of the arms 68, 69 results in acontinuously increasing pressure onto the head 3. The arms 68, 69 arefurther deformed outward during tightening of the locking member 8 andextend into the space provided by the first bore 51. When the arms 68,69 are in a second configuration, as shown in FIG. 20d , the head 3 isfully locked by the pressure exerted by the pressure member 6. Thesecond configuration is maintained even if the locking member 8 isadvanced further downward, such that the locking member 8 slides alongthe inner sides of the arms 68, 69, so that the advancement of thelocking member 8 relative to the arms 68, 69 may require only littleforce while the pressure member 6 is in the second configuration.

A loosening of the locking member 8 causes the arms 68, 69 to return tothe first configuration due to the high elasticity of the arms 68, 69.After loosening the locking member 8, the positions of the head 3 andthe rod 100 relative to the receiving part 5 can be adjusted again.

Further, a system can include the polyaxial bone anchoring device and atleast two rods 100, 101 having different diameters, where the boneanchoring device is configured to receive any of the rods (see, e.g.,FIGS. 17a, 17b ). FIG. 17a shows a polyaxial bone anchoring deviceaccording to an embodiment of the invention being used with a rod 100.If a rod of a different diameter than the rod 100 is used, for example,the rod 101 with a smaller diameter than the rod 100, the rod 101 canalso be safely supported on the rod supporting surface 64 of thepressure member 6 and clamped by the arms 74 a, 74 b, 75 a, 75 b of therod retention member 7, as depicted in FIG. 17b . Due to the smallerdiameter of the rod 101, the arms 74 a, 74 b, 75 a, 75 b extend to ahigher position on the rod 101 when viewed from the rod supportingsurface 64, which can compensate for a reduced friction hold whencompared to use with the rod 100 which has the larger diameter. Thesystem allows the surgeon to select an appropriate rod depending on thedesired application.

Further modifications of the above described embodiments are alsoconceivable. For example, the arms of the pressure member can have anyof many different shapes as long as a deformation from a firstconfiguration to a second configuration is possible. Even one single armmay be sufficient in some embodiments. In addition, the arms need not becontinuous pieces. For example, it is possible to construct the armsfrom two substantially rigid levers that are connected via a hinge toform a toggle lever. In this case, the deformation may involve, forexample, bringing the toggle lever from a first, angled configuration toa second, more substantially straight configuration. In someembodiments, the pressure member can be a monolithic piece that is madeof a material exhibiting high elasticity.

In another embodiment, the locking of the head is maintained by thepressure exerted by the pressure member in the second configuration,even if the locking member is loosened. Hence, further adjustments ofthe rod position are possible while the head still remains in the lockedangular position. In such an embodiment, the deformable portion can beadjusted back to the first configuration, for example, by applying aninstrument through the through-holes 58 c, 59 c in the receiving part 5.

The rod retention member can further have any shape that allows holdingof the rod. The arms of the rod retention member can be otherwiseshaped, and/or there can be less or more than four arms. In some cases asingle arm or clip may be sufficient. The rod retention member can alsobe formed as a monolithic piece with the pressure member. Also, in someembodiments, the rod retention member can be used in connection withbone anchoring devices that are of the top loading type, where the boneanchoring element is inserted from the top end.

In different embodiments, the pressure member can act via a directcontact onto the head, or may instead be operatively in contact with thehead via, for example, an additional intermediate part.

Meanwhile, for the bone anchoring element, different kinds of boneanchoring elements can be used, such as screws, nails, cannulatedanchors, hooks, etc. In some embodiments, the head and the shank may beconstructed from two pieces that connect to one another. Although thehead in the described embodiments is shown to have a spherical segmentshape, such that the bone anchoring element is polyaxially pivotablewith respect to the receiving part, in some embodiments, the head and/orthe receiving part or the pressure member may instead be designed suchthat pivoting can only take place, for example, in one single plane. Thepolyaxial bone anchoring device may also be designed such that largerpivot angles between the bone anchoring element and the receiving partin particular directions is possible.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

1. A coupling device for coupling a bone anchor to a rod, the couplingdevice comprising: a receiving part comprising a first end, a secondend, a longitudinal axis extending through the first end and the secondend, a channel at the first end for receiving a rod, and anaccommodation space for pivotably holding a head of a bone anchor; apressure member configured to be positioned in the receiving part and toexert pressure onto an inserted head to lock the head in the receivingpart, the pressure member comprising a deformable portion comprising afree end, wherein the deformable portion is configured to assume atleast a first configuration and a second configuration, and wherein thepressure member is movable along the longitudinal axis from an insertionposition to a locking position; and a locking member configured to beinserted into the channel to move in the channel along the longitudinalaxis and to exert a force onto the deformable portion of the pressuremember; wherein when the head is in the receiving part, and when thedeformable portion of the pressure member is in the first configurationand the locking member is in contact with the deformable portion at afirst axial position relative to the receiving part, the locking memberis moveable towards the head to exert a force on the deformable portionfor adjusting the deformable portion from the first configuration to thesecond configuration. 2-3. (canceled)
 4. The coupling device of claim 1,wherein when the head is in the receiving part, and when the deformableportion of the pressure member is in the second configuration and thelocking member is in contact with the deformable portion at a secondaxial position relative to the receiving part, the pressure memberexerts a force on the head that locks the head relative to the receivingpart.
 5. The coupling device of claim 1, wherein an opening is providedat the second end of the receiving part that is large enough to allowinsertion of the head from the second end.
 6. The coupling device ofclaim 1, wherein a first stop is provided to prevent the pressure memberfrom escaping from the receiving part in the insertion position.
 7. Thecoupling device of claim 1, wherein the pressure member can assume apre-locking position in which the head is prevented from removal fromthe receiving part.
 8. The coupling device of claim 7, wherein a secondstop is provided to hold the pressure member in the pre-lockingposition.
 9. (canceled)
 10. The coupling device of claim 1, wherein thedeformable portion is elastically deformable.
 11. The coupling device ofclaim 1, wherein in the first configuration the deformable portion isconfigured to protrude radially into a path of the locking member whenthe locking member is advanced into the channel.
 12. The coupling deviceof claim 1, wherein the deformable portion comprises one or more legs.13. The coupling device of claim 1, wherein the deformable portion isconnectable to a remaining part of the pressure member.
 14. A couplingdevice for coupling a bone anchor to a rod, the coupling devicecomprising: a receiving part comprising a first end, a second end, alongitudinal axis extending through the first end and the second end, achannel at the first end for receiving a rod, and an accommodation spacefor pivotably holding a head of a bone anchor; a pressure memberconfigured to be positioned in the receiving part and to exert pressureonto an inserted head to lock the head in the receiving part, thepressure member comprising a portion having a pressure exerting surfaceand a deformable portion comprising a free end, wherein the deformableportion is configured to assume at least a first configuration and asecond configuration; and a locking member configured to be insertedinto the channel to move in the channel along the longitudinal axis andto exert a force onto the deformable portion of the pressure member;wherein when the head is in the receiving part, and when the deformableportion of the pressure member is in the first configuration and thelocking member is in contact with the deformable portion at a firstaxial position relative to the receiving part, the locking member ismoveable towards the head to exert a force on the deformable portion foradjusting the deformable portion from the first configuration to thesecond configuration.
 15. A polyaxial bone anchoring device comprisingthe coupling device of claim 1 and a bone anchoring element comprising ashank for anchoring to a bone and a head, wherein the head is preferablyat least partially spherical.